1. Field of the Invention
The present invention relates to a nonreciprocal circuit device to be used in a high frequency band such as the microwave band, for example, an isolator or a circulator.
2. Description of the Related Art
Such an isolator is conventionally of the structure illustrated in FIG. 5 and FIG. 6. FIG. 5 is an exploded perspective view of the isolator, and FIG. 6 is a sectional view illustrating a fitting part of an upper yoke and a lower yoke.
In the isolator, a permanent magnet 3 is attached to an inner surface of a box-shaped upper yoke 12 as illustrated in FIG. 5 and FIG. 6, a U-shaped lower yoke 18 is mounted to form a magnetic circuit, a resin case 7 is arranged on a bottom wall 18a of the lower yoke 18. A magnetic assembly 5 in which three central conductors 51-53 are arranged with a ferrite 54, matching capacitors C1-C3, and a terminating resistor R are arranged in the resin case 7.
The upper yoke 12 and the lower yoke 18 are integratedly formed by punching a metallic sheet of a predetermined thickness consisting of magnetic metal, and respectively executing steps of bending, drawing, etc., to constitute a magnetic circuit forming the contour of the isolator and to play a role to protect the respective parts arranged in the magnetic circuit to be constituted by the upper and lower yokes 12, 18.
The upper yoke 12 is bent in a box-shape, and provided with an upper wall 12a and two pairs of side walls 12b, 12c, while the lower yoke 18 is bent in a U-shape, and provided with a bottom wall 18a and one pair of side walls 18b. Both end parts of a pair of side walls 12c opposite to each other of the upper yoke 12 form guide parts 12d, which project by a distance approximately equal to the thickness of the lower yoke side wall 18b from the outer surface of the side wall 12b. The guide part 12d plays a role to prevent the deviation in the horizontal direction (the direction of the arrow A in FIG. 6) of the lower yoke 18.
In the upper yoke 12 and the lower yoke 18, both end faces 18c of the lower yoke side wall 18b are lapped and fitted to each other along the guide part 12d of the upper yoke side wall 12c so that the surface of the upper yoke side wall 12b is brought into contact with the surface of the lower yoke side wall 18b, and the fitted parts are connected and fixed to each other by soldering.
The resin case 7 consists of a resin material with heat resistance and electric insulation. A side wall of rectangular frame shape is integrated with a bottom wall, input/output terminals 71,72, and ground terminals 73,73 are provided at predetermined positions, and the above-mentioned members of the isolator and the upper and lower yokes 12,18 are stably arranged and positioned.
In a nonreciprocal circuit device, any positional deviation of the yoke which constitutes the magnetic circuit adversely affects the electric characteristics. That is, when the upper and lower yokes are connected in a deviated position, the symmetry of the magnetic field distribution to be applied to a magnetic assembly is damaged by the deviation, and the electric characteristics such as isolation and insertion loss are degraded. Miniaturization of the mobile communication equipment in which this type of the nonreciprocal circuit device is employed has been rapidly developed, and further miniaturization of the nonreciprocal circuit device is strongly requested. In a recently miniaturized nonreciprocal circuit device (for example, 7.0 mm in length and width, and 2.5 mm in height), the deviation of the upper and lower yokes is required to be 0.1 mm or less from the electric and magnetic viewpoints.
However, in the conventional isolator, lapped parts of the guide part 12d of the upper yoke side wall 12c with both end faces 18c of the lower yoke side wall 18b are parallel surfaces to each other, and there has been a problem that the fitting work (assembly) becomes difficult if the clearance (gap) S between the guide part 12d of the upper yoke 12 and the end face 18c of the lower yoke side wall 18b is reduced in order to suppress the deviation of the lower yoke 18 in the horizontal direction. When the isolator is assembled by an automatic machine, a clearance S of not less than 0.1 mm is necessary for the automatic machine in the present situation, and the clearance S between the upper and lower yokes can not be set to be 0.1 mm or less.